In the industrial processes for the manufacture of vinyl chloride, 1,2-dichloroethane is cracked at elevated temperatures and pressures to form vinyl chloride and hydrogen chloride. The hydrogen chloride formed is used for the oxychlorination of ethylene to form additional 1,2-dichloroethane.
In the pyrolysis of 1,2-dichloroethane, acetylene is formed, generally in quantities of from about 0.05 to 0.5 mole %, and is entrained in the pyrolytically obtained hydrogen chloride. Since acetylene reacts under oxychlorination conditions, to form products which are difficult to separate from the reaction mixture, and which also impair the polymerization of vinyl chloride, it has to be removed.
The quantities of acetylene formed are, however, too small for their recovery to be economically worthwhile, or for conversion with subsequent separation. It has therefore been proposed to hydrogenate the acetylene impurities in the hydrogen chloride. The conversion products thus formed--ethylene and ethane--do not interfere in the oxychlorination process.
DE-OS 23 53 437 describes a hydrogenation process in which the hydrogenation is carried out at flow rates of from 7000 to 15,000 parts by volume of gas per part by volume of catalyst per hour (calculated at 15.6.degree. C. and 1 atmosphere), in at least two consecutive catalyst zones. In that case, the catalyst system has an activity profile that rises in the direction of the product flow.
That process of DE-OS 23 53 437 is, however, practicable only when used at relatively low pressures. After only a few months' operation, using an active catalyst at pressures of 8 bar absolute and above, leads to permanent deactivation of the catalyst as a result of the deposition of carbon black formed by acetylene decomposition.
DE-AS 15 68 679 proposes that the acetylene be removed from the hydrogen chloride by hydrogenation with platinum or palladium catalysts. The reaction parameters may vary within side limits; reaction temperatures of from 50.degree. to 200.degree. C., flow rates of from 300 to 5000 parts by volume of gas per part by volume of catalyst, hydrogen additions of from 2-fold to 5-fold molar excess (based on the quantity of acetylene), and pressures of up to 211 atmospheres absolute, but preferably from 1.4 to 7 atmospheres, are mentioned. Furthermore, according to the above-mentioned specification, supported catalysts based on any known carrier support material may be used.
It has been found however that, although in principle, operation under the conditions disclosed in DE-AS 15 68 679 is possible, the hydrogenation process can be carried out economically, over a relatively long period, only at pressures of up to a maximum of 7 bar absolute. Operation at higher pressures causes deactivation of the catalyst which results from the deposition of carbon black, and necessitates frequent change of the expensive noble-metal-based catalyst, loss of selectivity, with the resulting increased formation of ethane (instead of the desired ethylene), which is of no value, as well as increased formation of undesired hydrochlorination products of acetylene and ethylene.
On the other hand, in industrial processes for the manufacture of vinyl chloride, a pressure of 8 bar absolute and above is often used to react the hydrogen chloride in the oxychlorination stage (see DE-OS 14 93 213).